With the rapid advancement of laser technology, various kinds of lasers and laser devices have been extensively employed in numerous application fields. In particular, important progresses in material science have produced efficient coherent light sources with compactness, robustness and low cost. For instance, diode laser is a popular laser source because of its small volume, reduced power consumption, ease of mass production, and low manufacturing cost. To further expand the laser spectral and temporal quality, for instance, diode-laser pumped solid-state (DPSS) lasers are also playing key roles among coherent light sources. A DPSS laser employs one or plural diode lasers as its optical pump source, comprising laser gain medium that absorbs the pump diode laser energy and a laser resonant cavity that resonates the emission wavelength from the laser gain medium. In such a scheme, lasers with various wavelengths can be produced as desired by the choices of proper laser gain media.
Nonlinear optics allows optical frequency mixing to generate optical wavelengths not limited by atomic or molecular energy transitions in a laser host material. Therefore, wavelength-tunable coherent light sources can be built with the installation of an additional nonlinear crystal inside or outside a laser cavity. Usually, the size of a solid-state laser gain medium or a nonlinear optical material varies from millimeters to centimeters and thus the physical size of a DPSS laser with or without a nonlinear wavelength converter can be in the range of several centimeters. Furthermore, the beam quality, output power, and power stability of a DPSS laser are also greatly improved from those of a diode laser.
The research-and-development (R&D) progress of nonlinear-optics technology has provided unprecedented improvement to coherent light sources. To perform nonlinear wavelength conversion, intra-cavity and extra-cavity installations of the nonlinear optical material in a solid-state laser are the most common two schemes. In nonlinear wavelength conversion, phase-matching or wave-vector matching among mixing waves is often required, which is usually achieved in a birefringence crystal with the mixing waves polarized and incident in certain directions with respect to the crystal axes. Such a stringent phase-matching requirement usually leads to a fairly limited energy-conversion efficiency due to, for example, Poynting walk-off in a birefringence crystal or a non-ideal nonlinear coefficient at the phase-matching angle. In recent years, the so-called quasi-phase matching (QPM) technique has mostly lifted the above constraints by compensating the phase mismatch of mixing waves with a nonlinear optical material having a spatially modulated nonlinear coefficient. Such a QPM method allows a nonlinear-wavelength conversion process to access the maximum nonlinear coefficient of a nonlinear optical material, providing a better nonlinear conversion efficiency.
Generally speaking, the power of a continuous wave (CW) laser varies from several milliwatts to several watts. However, many important laser applications require high peak laser power within a certain laser pulse width. In particular, a high peak laser power favors nonlinear wavelength conversion. Second-order nonlinear wavelength conversion utilizes the second-order nonlinear susceptibility and in general an easier technique compared to a third-order nonlinear wavelength conversion. Among second-order nonlinear wavelength-conversion processes, an optical parametric process allows wavelength tuning but usually a much higher pump power than that for, say, second harmonic generation. Laser Q-switching is a common way of obtaining a high peak laser power from a laser source.
The working principle of a Q-switched laser is based on a technique in which the laser energy is accumulated in a time period comparable to the laser upper level lifetime and is released in an extremely short period of time to generate a high-power laser pulse. Thus, a high-quality laser Q-switch is crucial for a Q-switched laser source. Among miscellaneous laser Q-switching techniques, the EO laser Q-switching technique has a shortest switching time (on the order of tens of nanoseconds), a high timing accuracy, good stability, and excellent repeatability. However, the EO Q-switch in a typical Q-switched laser system is costly, bulky, and requires a nanosecond high-voltage pulse generator producing a few hundred to several thousand volts.
The present invention adopts a novel EO QPM nonlinear optical material as a laser Q-switch that has a much lower Q-switch voltage than a conventional EO Q-switch crystal such as potassium dihydrogen phosphate (KDP), potassium titanyl phosphate (KTP), lithium niobate (LN), etc., and thus allows a compact and low-cost design for a Q-switched laser system. When the QPM EO Q-switch is cascaded to a nonlinear wavelength converter, the intracavity Q-switched nonlinear wavelength conversion generates efficiency coherent optical radiations at desirable wavelengths. The laser system is particularly simple, if the QPM EO Q-switch is integrated into a QPM nonlinear wavelength converter in a monolithic nonlinear optical material. Such a system takes the full advantage of the lithographical-fabrication flexibility for a QPM nonlinear optical material.